A cooling system is an import link in the process of power generation. Steam exhaust which has done work needs to be condensed in a steam condenser, and recycled again.
There are two types of power station cooling systems, water-cooling and air-cooling, and the main difference there between lies in the cooling system. A power unit adopting an air-cooling system takes ambient air rather than water as a cooling medium of steam exhaust from a steam turbine.
There are two types of power station air-cooling systems. One type is indirection air-cooling systems, which can be divided into composite type air-cooling systems and surface type air-cooling systems. As in indirection air-cooling systems, cooling water from a surface type steam condenser of a steam turbine is cooled in a cooling tower. Another type is direct air-cooling systems (ACC). As for direct air-cooling, steam exhaust from a steam turbine is directly cooled by air, and the air and the steam are heat exchanged.
Direct air-cooling systems which have effectively solved the conflict between rich of coal and lack of water is a representative of the trend of future air-cooling systems (see, Tawney R, Khan Z, Zachary J. Economic and performance evaluation of heat sink options in combined cycle applications [A]. proceeding of Turbo Expo [C]. ASME/IGTI Turbo Expo, Atlanta, Ga., UAS, 2003).
The first 1500 KW direct air-cooling unit in the world was put into operation in 1938 in a mine-mouth power plant in Germany. Currently, Germany and USA are leading in the direct air-cooling technology. Existing direct air-cooling systems include German single tube, double tubes systems, and American single tube, triple tubes systems.
The principle of operation of a power station adopting direct air-cooling systems is shown as FIG. 1, wherein water is sent to a boiler by a pump, and is heated to overheat condition by burning fuel (coal, oil, natural gas or coal gas etc), and the over heat steam is sent by a pipeline to a steam turbine, which drives the power generator to generate power. The steam which has done work, whose pressure and temperature is reduced to 5 Kpa (kilopascal) ˜50 KPa and 30˜38°, is delivered by a pipeline into a direct air-cooling heat radiator, where the heat of the steam is carried away by a flowing air from bottom to top with the aid of a cooling fan and the steam is cooled to water. After collection, the cooled water is sent again by the pump to heating in the boiler, thereby a circulation is formed.
In recent years, China has introduced such technologies for applications in power generation industry, for example, Datong second power plant (2×600 MW (1000 kilowatt) unit) introduced a German single tube direct air-cooling system, Datong Yungang power plant (2×200 MW unit) introduced a German double tube direct air-cooling system, and Yushe power plant (2×300 MW unit) introduced an American triple tube direct air-cooling system.
Currently, there are more than one hundred direct air-cooling power units that have put into operation in China, and all of them are large scale units (300 MW or more per unit). A ratio of the installed capacity of direct air-cooling power units and that of indirection air-cooling power units has been over 9:1.
The core part of a direct air-cooling system lies in a fan cooling system, as is shown in FIG. 2. Core devices of a fan cooling system include a transformer, a frequency converter, electrical cables, a frequency variable electric motor, a speed reducer and a fan. The transformer, frequency converter, and electrical cables provide a required power supply. The electric motor dives the speed reducer, which dives the fan to rotate, so that air is driven to flow from bottom to top. When the flowing air passing through the heat radiator, a convection heat transfer is formed, thus the heat of the steam is carried away, and the objective that the steam is cooled is achieved.
In china, direct air-cooling systems are yet in preliminary stage, and experience in designing and running is insufficient. What interests the power plants owners more is the reliability of the air-cooling system, rather than the economy of the optimization of designing air-cooling systems (see, brief introduction of power station air-cooling systems, issued by China Industrial Control Website on Nov. 10, 2008). The independence designing and independence unitizing of large scale power station air-cooling systems have been one of the important tasks for the localization of significant technical equipments of China.
In summer, when the temperature is the highest, in order to ensure a normal operation of the units, improving the heat transfer capacity between the ambient air and the steam exhaust of the steam turbine is required to maintain the pressure of the steam condenser in the cooling system at a normal level. The most effective way to improve the heat radiating capacity of the heat radiator is to raise the flowing rate of the cooling air, i.e., to raise the head-on wind speed of the air-cooling heat radiator, thus the rotation rate of the fan is required to raise (see, Operation Problems and Solutions for Direct Air-Cooled Condenser System, Yang Lijun, Du Xiaoze, Yang Yongping, Liu Dengying, Guo Yuenian, Modern Electric Power, Vol. 23, No. 2, April 2006).
As shown in FIG. 2, The operation parameter of fan cooling systems in existing power station direct air-cooling systems at maximum output capacity are:
Transformer input voltage (a): 6 KV (kilovolt) or 10 KV, output voltage: 380V (volt);
Frequency converter output frequency (d): 55 HZ (Hertz);
Electric motor operation frequency (f): 55 HZ;
Speed reducer velocity ratio (g): (rotation rate of the electric motor at 55 HZ)/(110% of the rating rotation rate of the fan).
However, a commonly existed phenomenon is that, under high temperature in summer, the output and the efficiency of the direct air-cooling systems are well short from the designed requirements, which has a strong impact on the economic efficiency of the power plant.
TABLE 1data on power generation loss of several power plants in ChinaMaximumPowerScale ofoutput of thegenerationconstructionunit inloss (perNo.Power plant(MW)summer(MW)year, MW)remark1Datong2 × 6005002 × 900 ambient temperature: 30° C.;secondsteam machinery backpowerpressure: 30 KPaplant(II)2Yuncheng2 × 6004502 × 1350ambient temperature: 32° C.;power plantsteam machinery backpressure: 30 KPa3Tuoketuo2 × 6004502 × 1350ambient temperature: 26° C.;power plantsteam machinery backpressure: 30 KPa4Ural2 × 3002102 × 810 ambient temperature: 28° C.;Mountainssteam machinery backpower plantpressure: 30 KPaNote:“per year” in the table is calculated in accordance with the hottest three months in summer and three hours every day.